This invention relates to a moldable plastic wafer carrier or basket for silicon wafers which are designed for the transportation, storage and processing of silicon wafers used in the production of integrated circuit chips.
The processing of wafers towards integrated circuit chips requires that they be successively immersed, sprayed and/or rinsed with liquids or gases. Some of the chemical baths include various corrosive chemicals while certain of the baths are extremely hot, in the range of 180.degree. C. Wafers that are presently being processed have diameters as large as eight inches.
It should be recognized that silicon wafers are extremely delicate and brittle and may be only a few thousandths of an inch thick. The silicon wafers are extremely valuable and breakage of a wafer may represent a significant loss. Particulate and other contamination of wafers must also be avoided in order to manufacture integrated circuit chips operable within the range of standards set by the industry. Thus, there has been an increase in automated handling of the silicon wafers 5 axially aligned in batches supported in wafer carriers or baskets 10. Vacuum wands or fingers 57 are being utilized for handling individual silicon wafers after the carrier 10 is indexed, elevated and supported by wafer transfer equipment or devices 51. Robotic arms are also used for transporting the carrier 10 from station to station.
Typically, twenty-five of such wafers 5 are within a single wafer basket or carrier 10 which requires that a carrier be large enough for this capacity. Such a carrier, which is full of wafers, will easily weigh eight to 10 pounds. Two Hundred millimeter (200 mm) wafers are also now being used and will be in common use in the foreseeable future. Wafer carriers typically used to hold the silicon wafers are often conveniently formed of moldable plastic such as polypropylene, polyetheretherketone, PFA Teflon.RTM. (a registered trademark of E. I. du Pont Nemoirs company of Wilmington, Del.), perfluoroalkoxy-substituted polytetrafluoroethylene resin or other suitable plastics which are suitably inert, do not have significant out gasing characteristics, and will not corrode.
More specifically referring to FIGS. 1-4, the silicon wafers 5 may be seen within a wafer carrier 10 and in conjunction with known silicon wafer transfer equipment and devices 51 and 57. Wafer carriers 10 are disclosed in more detail in co-owned U.S. Pat. No. 4,949,848 issued on Aug. 21, 1990. Carrier 10 includes an open top 11, open bottom 12, upright sidewalls 13 with opposing lower wall portions 14 offset inwardly. Sidewalls 13 may include windows, cutaways or wash slots 16 along with opposing and inwardly projecting ribs 18 which have appropriately twenty-five wafer pockets or slots 19.sup.1 -19.sup.25 therebetween. Foot panels 20 join the lower wall portions 14 and support the carrier 10 when in its upright position. The top surfaces 22 of foot panels 20 support the wafers 5 within the carrier 10. At the top of sidewalls 13 are located flanges 28.
Carrier 10 has an H-shaped end wall 40 suitably with an H-shaped flange 42 which supports and adds strength to a horizontal indexing bar 44. When carrier 10 is stood on it H-shaped end wall 40, it is supported by H-bar contact rails or struts 45 each of which may have a robot pickup flange 46 and a contact surface 48. Some carriers may have machined nubs, knobs, or pads 49 on the contact surface 48. Carrier 10 also has an opposing end wall 50 opposite H-shaped end wall 40.
As stated, carrier 10 may be stood on its H-shaped end wall 40 with the indexing bar 44 used for properly indexing the carrier 10 on an elevator or a wafer transfer equipment device 51. The wafer transfer device 51 has a vertically movable positioning platform or plate 53 with carrier indexing positioners or webbing 55 thereon. By this arrangement, the indexing bar 44 may be properly located within the positioners 55 and the carrier 10 elevated upwardly or downwardly to position individual wafers 10 in front of a wafer removal or transfer device, such as a vacuum wand or finger 57. There are other end wall configurations for indexing but H-shaped end wall 40 is most common.
Due to the numbers of manufacturers of silicon wafers, wafer carriers, wafer transfer equipment, wafer processing chemicals and equipment, and integrated circuit chips, Semiconductor Equipment and Materials Institute, Inc. (SEMI) was formed and is located in Mountain View, Calif. 94043. SEMI, with industry participation, has set specifications and standards for wafers, carriers and equipment configurations. According to the SEMI standards, D1 is the tolerable distance from the contact surface 48 of the H-bar contact rails 45 to the centerline C.sub.L of the first wafer pocket 19.sup.1. Illustratively, D1 for a 150 mm (5.91 inches) carrier should be 0.5725 inches .+-.0.005 inches (14.54 mm.+-.0.13 mm). With respect to the wafer 5 and the first pocket 19.sup.1, the bottom edge of the wafer or the wafer plane W.sub.p -H must be 0.035 inches.+-.0.025 inches (0.89 mm .+-.0.635 mm) from the pocket centerline C.sub.L. Manufacturers of semiconductor processing materials and equipment who stay within these standards find that their materials and equipment are acceptable among wafer processors in that the silicon wafers 5 within carriers 10 are in predictable locations for suitable handling by processing equipment and devices 51 and 57.
When a wafer carrier 10 is molded or formed out of a plastic, such as PFA Teflon.RTM., the carrier during its molding, curing and setting up process is subject to forces and stresses due to the molten plastic location and associated shrinking forces which typically result in some degree of carrier warpage and/or shrinkage. Consequently, the carrier manufacturing industry has been required to have a narrow window of acceptable molding process parameters including temperature, pressure and time in an effort to minimize carrier warpage and shrinkage and to maintain strict industry and/or SEMI wafer carrier standards.
Depending upon the particular configuration and plastic material, many molded carriers experience shrinkage and warpage to a degree that they are unfit for use and are rejected. Rejection of some carrier configurations difficult to mold maybe in a range of up to 50 percent. Rejection may occur when the particular carrier does not fit within the SEMI standards, experiences a "rocking chair" effect in that it does not sit steadily flat upon the wafer transfer equipment 51, or experiences a wafer tilt. Water tilt means that the wafers are not oriented true to horizontal but rather may have their top edges, when in the indexed carrier, tilted downwardly. Equipment access to wafers in this condition often results in breakage and contamination.
There is a need for a moldable plastic wafer carrier with a wide window of acceptable molding process parameters. Such a carrier should readily compensate for the stresses and forces which result in carrier warpage and shrinkage during its formation. The carrier should compensate for wafer tilting and should not rock when standing on its H-shaped end wall. Although some warpage and shrinking may be inevitable, the carrier should have compensating features which are not susceptible to the deleterious effects of warpage and shrinking. The carrier should be predictably and steadfastly locatable upon wafer transfer equipment as its supports and carriers wafers. Finally, such a carrier should not be subject to a measurable range of rejection due to nonconformance with industry accepted standards and SEMI specifications.